Tuesday, March 17, 2009

The Alchemy of Zinc

Back in the day, alchemists apparently spent all their time attempting to turn base metals into gold. I say apparently, since that is the common perception of alchemists. In reality, most alchemists were devoted to what was often referred to as the “Great Work.” According to alchemical theories, much of what is created by nature is imperfect, and by applying the art of alchemy, natural substances can be brought to a higher state of perfection. Since gold was the height of perfection for metals, it was thought that base metals could be transmuted into gold merely by removing these imperfections. The Philosophers Stone wasn’t just about making gold or the “Elixer of Life”, it was a process/device/concept for bringing about perfection.

Color was very important to the alchemists, perhaps since it was one of the few clues with which they had to go on during their experiments. Colors and color changes were rigorously recorded and eventually incorporated into many of the alchemical theories. Certain color sequences were expected during the path towards perfection. In the transmutation of base metals into gold, for example, a color sequence of black to white to yellow to purple was thought to be required. Later theories redefined the sequence as black to white to red. While this fascination with colors may have led them astray on occasion, it has also led to a vast array of wonderful, full color illustrations, intricately drawn and full of alchemical symbolism. Here is one such example.

An experiment often touted as reminiscent of alchemy and demonstrating the relative ease of generating color changes involves the apparent conversion of a copper penny into silver or gold. Simply dissolve some Zn powder in warm NaOH solution, toss in a penny, and watch the penny turn silver as a coating of Zn forms on its surface. Heating the penny turns it gold as the Zn and Cu alloy to form brass. (My kids were duly impressed when I performed this experiment for them, informing me that it was “awesome”, but considering they said the same thing when I turned phenolphthalein red, the “awesomeness” bar may be set pretty low.)

But why does this work? I’ve coated pennies with silver and mercury before, but zinc is more electropositive than copper. Based on electrochemical potentials, zinc shouldn’t plate out on copper -- copper should be plating out on zinc. Apparently I’m not the only person to wonder about this, as web pages devoted to this effect can be found throughout the Internet, for example, here and here.

First of all, Zn dissolves in NaOH to make zincate ion and hydrogen.

Zn + 2OH- + 2H2O ----> Zn(OH)42- + H2

Okay, that’s straightforward chemistry. But how does the copper reduce the Zn(OH)42-? The answer is that it doesn’t. Surprisingly, copper is not the reductant . Copper is not oxidized and it does not go into solution. Copper’s role is to create a galvanic cell with the undissolved portion of the zinc powder. Zinc does not plate out on the penny until the copper is in direct contact with zinc metal. It turns out that zinc metal is the reductant, which sounds bizarre, at least to me. Once the zinc and copper metals are in contact and the galvanic cell is created, the zinc begins to oxidize…

Zn + 4OH- ----> Zn(OH)42- + 2e-

And its electrons are now available to reduce the zincate ion near the surface of the penny...

Zn(OH)42- + 2e- ---> Zn + 4OH-

So the overall reaction is:

Zn + Zn(OH)42- ----> Zn(OH)42- + Zn

Basically there is no net reaction! Which means the zinc is essentially just migrating from the surface of the zinc powder to the surface of the penny. At first glance, this would appear to violate the laws of thermodynamics, but obviously it doesn’t. I assume there is some sort of entropy effect here, perhaps related to the galvanic cell, but I don’t know the exact cause. In any case, this reaction appears just as mystical as alchemy itself!

If anyone happens to know any more about this process, I’d be happy to hear from you.